1. Field of the Invention
The present invention relates to a method for manufacturing a preform for optical fibers by a vapour deposition process, and a method for forming optical fibers in which the optical preform is heated at one end from which optical fiber is subsequently drawn.
2. Background of the Invention
Methods for manufacturing a preform for optical fibers by means of a vapour deposition process, and methods for forming optical fibers in which an optical preform is heated at one end from which an optical fiber is drawn are described in Dutch Patent NL 1 023 438.
Preforms for optical fibers are formed by means of an internal chemical vapour deposition technique (CVD), in particular plasma chemical vapour deposition (PCVD), wherein reactive glass-forming gases, possibly doped, are reacted inside a hollow glass substrate tube, resulting in the deposition of one or more glass layers on the inner surface of the hollow substrate tube. Such reactive gases are supplied on one side of the substrate tube, in other words, the supply side, and as a result of the special process conditions they form glass layers on the interior of the substrate tube. An energy source is moved back and forth along the length of the substrate tube for forming the glass layers. The energy source, in particular a plasma generator, supplies high-frequency energy, thereby generating a plasma in the interior of the substrate tube, under which plasma conditions the reactive glass-forming gases will react (the plasma CVD technique). It is also possible, however, to supply the energy in the form of heat, in particular by means of burners, on the outer side of the substrate tube or via a furnace that surrounds the substrate tube. The aforesaid techniques have this in common, that the energy source is moved back and forth with respect to the substrate tube.
A drawback of the techniques described above is the fact that defects may develop in the deposited layers near the reversal points as a result of the reciprocating movement of the energy source. Such defects are called “taper,” in which context a distinction is furthermore made between geometric taper and optic taper. The term “geometric taper” is understood to mean that the thickness of the total deposition, in other words, of all the glass layers, is not constant along the length of the tube. The term “optic taper” is understood to mean that the optical properties, which are mainly determined from the fiber that is eventually drawn, are not constant along the length of the preform. Optic taper is to a minor extent caused by deviations in the layer thickness, but mainly by deviations in the refractive index, or refractive index profiles, along the length of the preform. Besides an adequate control of the geometric taper, it is also desirable that the deviations in the refractive index contrast A be as small as possible over a maximum length of the preform so as to realize an adequate control of the optical properties of the fibers to be formed.
A disadvantage of taper is the fact that the useful length of the preform is limited, which means that a smaller amount of fiber can be obtained from one preform. In addition to that, the properties of an optical fiber may not be constant along the length of the fiber due to said taper.
U.S. Pat. No. 4,741,747 relates to a method for manufacturing a preform for optical fibers, in which the reduction of so-called end taper is intended to take place by moving the plasma nonlinearly as a function of time in the region of the reversal point and/or by varying the intensity of the plasma along the length of the glass substrate tube.
European patent application No. 0 038 982 relates to a method for manufacturing a preform for optical fibers, in which the plasma generator is moved along the length of the substrate tube, which plasma generator produces a hot zone, such that said hot zone can be regarded as a so-called “tandem hot zone” comprising at least two zones, in other words, zone I and zone II.
European patent application No. 0 333 580 relates to a method for manufacturing preforms for optical fibers, in which a variable power microwave generator is used, but in which no use is made of a non-isothermal plasma that is moved back and forth between two reversal points along the length of the substrate tube.
British patent publication GB 2 118 165 relates to a method for manufacturing a preform for an optical fiber in which the velocity of the heat source axially along the substrate tube is in accordance with a specific mathematical equation, said velocity being a function of the position of said heat source along said tube, so that the total deposition thickness of the glass layers is claimed to be substantially constant along the length of said tube.
U.S. Pat. No. 5,188,648 granted in the name of the present applicant provides a method for manufacturing a preform of optical fibers in which the movement of the plasma is interrupted each time the plasma reaches the reversal point near the gas inlet point of the substrate tube, whilst the glass deposition continues, where interruption of the plasma movement lasts at least 0.1 seconds.
It has been found that soot deposition takes place at the supply side of the substrate tube during the deposition step, which soot deposition manifests itself as a ring on the inner surface of the hollow substrate tube, which ring also extends along a specific length of the substrate tube. It is assumed that such soot deposition takes place as a result of the relatively low intensity of the plasma in the region where the deposition takes place. It is further assumed that the temperature on the inner surface of the hollow substrate tube at the beginning of the deposition process plays an important part in the formation of such a soot ring. The presence of such a soot ring will have an adverse effect on the effective length of the preform. After all, the region of the soot ring in the substrate tube cannot be used for forming an optical fiber therefrom which meets the required product specifications. Another drawback of the so-called soot deposition is the fact that if a number of soot rings more or less overlap, there is a significant risk of fracture of glass layers, which means a loss of preform rod, which is undesirable.